Rear projection screens and light diffusers are light filters which provide an optically diffusing medium for transmitting light from an image source on one side of the screen to a viewer on the opposite side of the screen. A basic refractive light filter has been described in U.S. Pat. No. 2,378,252, which includes a refracting lens system as its principal component. The refracting lens system comprises an array of spherical glass or resin beads embedded in an opaque binder layer and mounted on a transparent support material. Typically, the light filter is oriented with the bead layer towards the image source and the transparent support material towards the viewers. U.S. Pat. No. 3,552,822 discloses a similar light filter which also includes an anti-reflection coating and has its bead layer oriented away from the image source.
The opaque binder layer serves a number of purposes, including affixing beads to the support material, reducing the reflectivity of the filter, and reducing the amount of light transmitted through the interstices between the beads of the lens system. Light from an image is refracted by the beads and dispersed to the viewer through a transmission area of the beads. This transmission area comprises the point of contact between the bead and support material and the area surrounding this point where the binder layer is too thin to absorb the refracted light.
Rear projection screens and light diffusers are characterized by their ambient light rejection, resolution, gain, and contrast, properties which are determined by the structure and composition of the component materials. For example, the gain which is a measure of the intensity of transmitted light as a function of the viewing angle, is determined by the index of refraction of the spherical beads and the surrounding medium. Similarly, the ambient light rejection and contrast of the light filter are determined largely by the opacity of the binder layer. The resolution of the screen is determined by the size of the beads used in the lens system.
However, the interdependence of certain optical properties and their dependence on the properties of component materials, limit optimization of the optical properties of basic refractive light filters. For example, if the opacity of the binder layer is increased to enhance the ambient light rejection of the viewing surface, transmission of refracted image light through the binder layer in the transmission area of the bead will be reduced. In addition, the range of indices of refraction of available materials also limits the performance of single layer filters. Such interdependencies and material limitations hamper the performance of basic refractive filters.